Remember how I said that George's teeth have never offered to make contact with the mares? Never say never.
A day or two ago, I saw Rose nibbling on some black walnut leaves overhanging their field. I believe black walnut is not great for horses (although we never had a horse want to eat it before), so I shooed her away. George picked that moment to chase her. I thought it was a coincidence.
This morning, she went to eat some of the leaves again. I shouted anxiously at her to leave them alone. George ran at her and gave her a definite, teeth-bared, bite on the rump (no damage done).
I was rather impressed and thanked him.